Much of current thought on the function of the primate visual system centers on the idea that separate pathways or channels are active in the detection and analysis of color, form and depth. For the dorsal lateral geniculate nucleus (LGN), two anatomically and physiologically distinct groups of layers (parvocellular and magnocellular) make up the thalamic component of those pathways. Previous work in this lab has focussed on a third, distinct population of neurons in the monkey LGN. The neurons, which occupy the LGN intercalated layers, display a unique neurochemistry (immunoreactivity for the calcium-related proteins, calbindin and CaM II kinase) which permits their study in isolation from the more numerous and larger cells of the magno- and parvo-cellular layers. This population will be examined in the studies outlined in the present proposal. Four groups of studies are proposed, which will involve: 1) Complete neurochemical characterization of the intercalated neurons to determine whether subpopulations of these cells exist and whether they overlap to any extent with populations that can be identified as inhibitory interneurons; 2) Progressively finer resolution of their cortical projections to determine if they send axons to the primary visual area of cortex (VI), most specifically to the periodic patches or blobs of cytochrome oxidase staining in layers II and II, which have been characterized as part of a color channel; 3) Quantitative analyses using a modem stereological method to determine whether the intercalated neurons are sufficiently numerous to account for a major projection to V1; 4) Evaluation of a unique visual drive onto these neurons as indicated by the termination of two groups of axons, one that is a morphologically distinct population of retinal axons and the other, the axons arising from the superior colliculus. Together, these studies will provide the neurochemical, morphological and connectional basis for a re-evaluation of geniculocortical organization. By focussing upon chemical characteristics of distinct groups of neurons in the primate visual system, these studies are likely to provide useful tools for the study of the visual system in normal humans and in patients suffering from neuropathologies which interfere with normal vision.